In recent years there has been an upsurge of interest in the electric automobile due to pollution problems with internal combustion engines and, also, because of the energy crisis. Accordingly, there has been a parallel increase in attempts at developing batteries or cells suitable for powering electric vehicles.
Lead-acid batteries have been used in the past for electric automobiles and are currently in use for dollies and vehicles used in factories. However, the high weight and expense which result from providing a sufficient number of batteries to achieve a practical range of operation for an electric automobile is too great to make such vehicles acceptable to the general public.
Prime candidates to replace the lead-acid battery are the Ag-Zn and Ni-Zn alkaline batteries. However, it has been found that the nickel electrodes of such batteries are subject to warping after relatively short usage. This requires that the separators between the nickel and zinc or cadmium electrodes be highly flexible so as to prevent cracking and disintegration of the separator and the consequent shorting of the electrodes.
Another objective which must be met with regard to Ag-Zn and Ni-Zn alkaline batteries is the reduction of cost to an acceptable level. Because the cost of the separators is a significant portion of the total cost of an Ag-Zn or Ni-Zn alkaline cell, a reduction in the cost of manufacturing separators is an important factor. Thus, a separator which utilizes low cost materials and which is easy to manufacture is desirable.
Flexible polymeric battery separators are well known in the art. The microporosity needed for ionic transport of the electrolyte is typically achieved by leaching out a component from a film formed from a homogeneous mixture of at least one polymer, and at least one particulate filler. Solvents or plasticizers have also been leached from the film to create porosity. The leaching has been performed in-situ by the electrolyte or out of the cell by a leaching solvent. Porosity has also been controlled by the relative amounts of filler and polymer.
In U.S. Pat. No. 3,551,495 a polyolefin having a molecular weight above 300,000 is admixed with a plasticizer and an inert filler and extruded into a sheet. The sheet is treated with a solvent to dissolve the filler and/or the plasticizer thereby creating a microporous sheet. The size of the filler particles range from an average of about 0.01 microns to about 10 microns in diameter depending upon the porous character of the filler. The surface area of the filler can range from about 30 to 950 square meters per gram, preferably at least 100 square meters per gram.
In U.S. Pat. No. 3,713,890 a latex-type polymer is admixed with an inorganic particulate filler which is insoluble in water and also insoluble in alkali. A film is cast from the mixture, and then the film is dried and sintered. The sintered film is then treated with alkali, preferably aqueous KOH, to substantially increase the ionic conductivity of the film, without substantial dissolution of the filler particles. The filler particles, it is disclosed, should have a sufficiently fine particle size, preferably not greater than about 10 microns, to permit uniform distribution of the filler particles throughout the film of polymeric binder matrix. The proportions of polymeric binder and filler are such as to result in a film having a substantial concentration of the filler. The mixture, or slurry, used in forming the film generally contains about 50% to about 95% filler and about 5% to 50% polymer, by weight of total solids. The aqueous alkali treatment of the film, it is disclosed, creates microporous regions for ionic conduction both at the interface between the filler and the binder as well as between the particles of filler. The microporosity is apparently created by the breaking of surface bonds between the binder and the filler particles and between adjacent filler particles.
In U.S. Pat. No. 3,730,777, a porous polymeric battery separator for a dry cell is obtained by admixing a resin which is soluble in the aqueous electrolyte with an insoluble resin and with a filler. The mixture is formed into a film. The film is made microporous in-situ by dissolution of the soluble resin in the electrolyte. Swelling of the soluble resin in the film due to the electrolyte can also provide the porosity. The filler has a particle size of less than 50 microns and is used in an amount of up to 24 times the total weight of the resins. The soluble or leachable resin does not have to be selected to avoid carbonate formation or degradation on recharging because the separator is for a primary cell.
U.S. Pat. No. 3,749,604 discloses a battery separator designed to prevent silver ion flow and to resist highly branched zinc dendrite formation in alkaline silver oxide-zinc secondary batteries. The separator is obtained by coating a flexible porous support with an alkali-resistant, water insoluble polymer, inert, organic filler particles and a water soluble organic solvent. Porosity is achieved by removing the solvent with a water-acid or water-organic solvent extracting solution. The filler particle size can vary from about 74 to 700 microns, preferably 149 to 700 microns. The filler polymer ratio in the separator is between about 1:1 to 5:1. High filler loading is preferred (about 3 parts filler to 1 part polymer) to provide a large number of filler contact points.
In U.S. Pat. No. 3,861,963 porosity in a battery separator for an alkaline battery is achieved by admixing a polymer, a solvent for the polymer, particulate inorganic fillers, and potassium titanate, removing the solvent, and curing the polymer. The polymer is one which can bond the filler particles and the potassium titanate together upon curing. However, it does not fill the voids between the filler particles and potassium titanate particles so as to result in a porous structure. The ratio of the sum of the amounts of filler and potassium titanate to the amount of polymer is preferably at least 1:1. Amounts of polymer over 50% of the mixture, it is disclosed, increase the flexibility of the separator at the expense of increased internal resistance. The filler particle size, it is taught, should be such that 95% of the particles have a size of less than 10 microns.
In U.S. Pat. No. 4,085,241 to the present inventor, a flexible porous battery separator is obtained by coating a flexible porous substrate with a slurry comprised of a copolymer or rubber based resin, a plasticizer, two different particulate fillers and an organic solvent. One of the filler materials is inert to the alkaline electrolyte and the other filler material is reactive with the alkaline electrolyte. Preferably, the plasticizer is one which reacts with the alkaline electrolyte to produce a short chain alcohol or glycol.
The reactive fillers of U.S. Pat. No. 4,085,241 are selected from the group consisting of calcium silicate, silicon dioxide (silica) and alumina having a particle size of from 0.01 microns to 3 microns. When incorporated in the other unreactive or inert filler material(s), they react with the alkaline electrolyte to form pores in the separator coating. The inert filler particles, it is disclosed, have a particle diameter from about 0.1 micron to 10 microns. The combined volume percent of the inert and reactive filler materials is from 25% to less than 50% by volume of the separator material formation. The rubber based resin or copolymer comprises between 50 and 80% by volume of the separator coating material.
In U.S. Pat. Nos. 3,551,495, 3,713,890, 3,749,604, and 3,861,963, creating the needed microporosity out of the cell involves extra, costly processing steps prior to insertion of the battery separator into the cell. Further, in U.S. Pat. No. 3,713,890, breaking of the bonds between the filler particles and the binder by the KOH treatment can cause subsequent loss of the filler particles and large voids upon warpage of the battery electrodes.
In U.S. Pat. Nos. 3,730,777 and 4,085,241, the microporosity is created in-situ by the electrolyte, thereby reducing the number of processing steps prior to insertion of the battery separator in the cell. However, in U.S. Pat. No. 3,730,777 the high filler content and the removal of the resin from the film by the electrolyte results in a film of low flexibility. Also, pore size distribution is difficult to control because it depends upon the leaching of a resin from a mixture of resins. It has been found that in U.S. Pat. No. 4,085,241, the use of the reactive fillers as pore formers makes pore size control and distribution difficult.
According to the present invention, there is provided a battery separator for alkaline batteries which is sufficiently flexible to withstand stresses produced by warping of the nickel electrodes in Ni-Zn alkaline batteries. The present invention provides a method for obtaining controlled pore size and uniform pore size distribution without the need for costly, time-consuming out-of-the-cell pore-forming treatment steps.